Atmospheric circulation patterns explaining climatological drought dynamics in the boreal environment of Finland, 1962-2011

Understanding the atmospheric circulation patterns (ACPs) influencing drought development is important in management of this spatially extensive and recurrently prolonged natural hazard. Using the Standardized Precipitation Evapotranspiration Index (SPEI), this study evaluates the spatio-temporal variability in climatological drought dynamics (in terms of intensity, frequency, duration, and extension) in Finland in 1962-2011. The SPEI values are calculated in 12-, 3-, and 1-month time steps (SPEI-12, -3, and -1, respectively) and the analyses are based on annual (SPEI-12Ann), seasonal (e.g. SPEI-3Win for winter), and monthly (e.g. SPEI-1Dec for December) time scales. The relationships between SPEI values and well-known ACPs of climate variability over Finland are also investigated. Our analysis shows that on national scale, the longest observed drought is 12 months (October 2002-September 2003) observed in SPEI-12 values. Droughts intensify in April, August, and September, and largely occur over the south, centre, east, and west of Finland. December has the highest frequency of droughts, but with significant wetting trends. Analysis of annual SPEI-12 indicates an increase in the probability of drought extent over time. In warm months (April-September), influential ACPs for temperature variability and drought conditions are mainly the Arctic Oscillation (AO) and the Scandinavia pattern. For the cold months (October-March), the AO, the North Atlantic Oscillation, the East Atlantic, and the Polar patterns influence precipitation deficit and SPEI. Although Finland has experienced wetting trends during the period 1962-2011, this study provides a detailed spatial assessment of climatological drought variability and the plausible atmospheric circulation drivers for the boreal environment of Finland in recent decades.